1. Field of the Invention
The present invention relates to liquid crystal displays. More particularly it relates to active matrix liquid crystal displays (AMLCD) having well-aligned pixel electodes, and to a method of fabricating such active matrix liquid crystal displays.
2. Discussion of the Related Art
An active matrix liquid crystal display is typically fabricated by joining an upper plate to a lower plate, and then injecting a liquid crystal material between the plates. A lower plate usually includes a plurality of pixel cells that are formed from switching devices (usually thin film transistors) and pixel electrodes. Such lower plates further include a plurality of drive lines that connect drive signals to the pixel cells. An upper plate usually includes a plurality of color filters and a common electrode. To complete an active matrix liquid crystal display, polarizing plates are attached to the upper and lower pirates.
FIG. 1 schematically illustrates a typical prior art active matrix liquid crystal display. As illustrated, data lines, including a data line 15L, cross a plurality of gate lines, including gate lines 11L and 10L. The areas between the data lines and the gate lines define pixel cell regions. A thin film transistor (hereinafter abbreviated TFT) that acts as a switching device is formed at intersections between the data lines and the gate lines.
A TFT includes a gate electrode 11G, which is a protrusion from the gate line 11L, a source electrode 15S, which is a protrusion from the data line 15L, a drain electrode 15D, and an active layer 13. The active layer is overlapped by the electrodes. As shown, a pixel electrode 17 connects to the drain electrode 15D.
Prior art active matrix liquid crystal displays are usually fabricated using photolithograpy. For example, to form the data line 15L and source electrode 15S, the gate line 11L and gate electrode 11G, and the drain electrode 15D a metallic layer is deposited on a prepared substrate. The deposited metallic layer is then coated with a photoresist layer. The deposited metallic layer is then patterned by selectivly exposing the photoresist layer through a prepared mask using a light source that is above the metallic layer. The exposed photoresist layer is then etched to leave metallic conductors for the lines and electrodes. Pixel electrodes are then formed in the same manner. However, pixel electrodes are typically fabricated after the lines and electrodes. Significantly, the pixel electrodes are fabricated from a transparent material.
While the photolithographic process described above has proven useful, it has problems. One particular problem when fabricating prior art active matrix liquid crystal displays is the likelyhood of misalignment of the pixel electrodes relative to other features. Such misalignment may be caused by misalignment of exposure masks or of the exposure apparatus, or by an etch deviation due to etch conditions.
FIG. 2 assists the understanding of pixel electrode misalignment by showing a cross-sectional view taken along the line I-I′ of FIG. 1. Initially, a gate insulating layer 12 is formed on a substrate 100. The data line 15L is then photolithographically formed on the gate insulating layer 12. A protection layer 16 is then formed over the structure. Pixel electrodes 17 are then photolithographically formed on the protection layer 16. Ideally, the data line 15L is centered between the pixel electrodes such that the intervals L and R are the same. Unfortunately, the locations of the pixel electrodes can deviate from their intended locations. Such deviations can be caused divisional exposure.
With divisional exposure, each exposure step requires new exposure equipment, such as a photomask, to be set-up. Thus, it is very difficult to control the intervals L and R such that they are even. As a result, image defects referred to as image stains are created. Furthermore, cross-talk between the pixel electrodes and the data lines becomes more severe due to deviations of parasitic capacitances.
Therefore, an improved active matrix liquid crystal display, and a new method of fabricating such an active matrix liquid crystal display, having accurately positioned pixel electrodes would be beneficial.